"After nearly three years of work, I have a pleasure to announce that Qubes 1.0 has finally been released! [...] I would like to thank all the developers who have worked on this project. Creating Qubes OS has been a great challenge, especially for such a small team as ours, but ultimately, I'm very glad with the final outcome - it really is a stable and reasonably secure desktop OS."

Considering it is based on Linux I can see why they thought this solution was the only reasonable one.

If each process would have it's own mount table (and in turn it's own view of the filesystem, including multiplexing of resources) as in Plan9 and processes could only communicate through the filesystem and not through some obscure system calls there would not have been any need for this what so ever because that together with the MMU would been enough. Chroot, as pointer out earlier would not have been a reasonable alternative either.

The overhead of the solution they came up with must be incredibly high. It is an ugly workaround, but it's nice they made it work anyway.